In a liquid crystal display device using such a driver circuitry substrate provided with switching devices or so-called a TFT substrate, TFTs are formed on an insulating substrate made of a glass or a quartz, and it is being used for switching pixels and other driving circuits. As for the TFT substrate, recent technology requires much more functions for the TFT substrate by providing plural types of TFTs having different breakdown voltage capability characteristics on a single insulating substrate. Specifically, at least two types of TFTs are needed for the TFT substrate. One is a TFT driven at relatively low voltage (about 1.5-5 V) with high speed which is often used for a signal processing circuit or the like. The other is a TFT driven at relatively high voltage (about 10-40 V) which is often used for driving a pixel or a peripheral circuit.
In general, it is difficult to form such a TFT which is compatible for having a high current driving capability and a high breakdown voltage capability. Thus, different types of TFTs are formed separately on the same substrate such that one TFT having the high current driving capability and the other TFT having a high breakdown voltage capability are individually fabricated. For example, as indicated in FIG. 25 and paragraphs 118-124 of Japanese Patent Application Laid-Open No. 2003-45892 (Patent Document 1), a low voltage driving TFT and a high voltage driving TFT are designed to have different thickness of the respective gate insulating films between top gate electrodes and semiconductor layers. In such structure, the low voltage driving TFT is provided with a first gate insulating film on an upper part of the semiconductor layer such as a silicon layer while the high voltage driving TFT is provided with not only the first gate insulating film but also a second gate insulating film on an upper part of the semiconductor layer so that a total thickness of the gate insulating film of the high voltage driving TFT becomes a total sum of the thickness of the first gate insulating film and the second gate insulating film.
In the Patent Document 1, the low voltage driving TFT and the high voltage driving TFT require individual process to form each top gate electrode, respectively. In addition, the first and the second insulating films are respectively formed prior to forming the gate electrodes for the low voltage driving TFT and the high voltage driving TFT. Accordingly, such structure is disadvantageous in view of a production cost by requiring much more fabricating process.